1. Field of the Invention
The present invention relates to start control of a passenger protection system and, more particularly, to an improvement in start control of a two-stage passenger protection system which has two inflators and is constructed to fire them in order under predetermined conditions.
2. Description of the Related Art
In the prior art, in the passenger protection system which is represented by the air bag system, for example, such an air bag system that inflates an air bag at a time when a impact acceleration in excess of a predetermined reference is detected has been the mainstream. However, according to various later studies, experiments, etc., it becomes apparent that in some cases it is not always proper for the passenger protection to inflate the air bag at a stretch.
In recent years, various two-stage air bag systems have been proposed from a viewpoint of passenger protection (for example, see Patent Application Publication (KOKAI) Hei 2-310143, etc.). In such two-stage air bag systems, a first stage inflator is fired under predetermined conditions to inflate the air bag until a predetermined size, and then a second stage inflator is fired to inflate the air bag up to its maximum at a point of time when second predetermined conditions are satisfied.
In such two-stage air bag systems, it becomes an issue from a viewpoint of proper passenger protection how the first stage inflator and the second stage inflator should be fired.
As a relatively simple method of firing the inflators, for example, the start control can be thought of such that a deceleration caused in a crash is measured so as to calculate its integral value and then the first stage inflator and the second stage inflator are fired respectively when such integral value exceeds respective threshold values.
Such start control can be employed enoughly enoughly in practical use unless so-called stagnation of the integral value of the deceleration is caused between the fire of the first stage inflator and fire of the second stage inflator.
However, according to the type of the crash, sometimes such a phenomenon is produced that, since increase in the integral value of the deceleration becomes small, i.e., a stagnation state is generated after the first stage inflator has been fired, the integral value of the deceleration hardly exceeds the threshold value to generate firing of the second stage inflator, although fire of the second stage inflator is requested. For example, in the case of the so-called offset crash, because a part of constituent parts of the vehicle is damaged by the impact, sometimes the integral value of the deceleration is stagnated after the first stage inflator has been fired. In the above type of crash, the case is sometimes caused where, only if fire of the inflators is determined based on the decision whether or not the integral value of the deceleration exceeds the threshold value being set under aforementioned simple conditions, the second stage inflator cannot be fired in a period of time when fire of the second stage inflator is truly needed.